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LEVELS OF SEDATION

Anesthesia is a continuum from consciousness to general anesthesia. The recognized levels of sedation are: minimal sedation (formerly referred to as anxiolysis), moderate sedation/analgesia (formerly referred to as conscious sedation), deep sedation/analgesia, and general anesthesia. The patient’s responsiveness, airway maintenance, spontaneous ventilations, and cardiovascular function categorize these different levels or depths of sedation.

Minimal sedation is defined as a “drug induced state during which patients respond normally to verbal commands. Although cognitive function and coordination may be impaired, ventilatory and cardiovascular functions are unaffected.”

Moderate sedation/analgesia (which we will refer to as moderate sedation) is defined as a “drug induced depression of consciousness during which patients respond purposefully to verbal commands, either alone or accompanied by light tactile stimulation. No interventions are required to maintain a patent airway, and spontaneous ventilation is adequate. Cardiovascular function is usually maintained.”

Deep sedation/analgesia (which we will refer to as deep sedation) is defined as a “drug induced depression of consciousness during which patients cannot be easily aroused but respond purposefully following repeated or painful stimulation. The ability to independently maintain ventilatory function may be impaired. Patients may require assistance in maintaining a patent airway, and spontaneous ventilation may be inadequate. Cardiovascular function is usually maintained.”

General anesthesia is defined as a “drug induced loss of consciousness during which patients are not arousable, even by painful stimulation. The ability to independently maintain ventilatory function is often impaired. Patients often require assistance in maintaining a patent airway, and positive pressure ventilation may be required because of depressed spontaneous ventilation or drug induced depression of neuromuscular function. Cardiovascular function may be impaired.”

These levels of sedation are not dependent on the route of drug administration nor the specific anesthetic agent or combination of agents administered. For example, given a large enough dose or a more typical dose in a medically compromised patient, an oral benzodiazepine could induce a level of moderate sedation/analgesia to deep sedation/analgesia. Various oral agents (e.g., benzodiazepines or chloral hydrate) when co-administered with nitrous oxide have produced more profound levels of sedation inducing deep sedation/analgesia or general anesthesia. Alternatively, propofol and methohexital, which are commonly associated with deep sedation/analgesia and general anesthesia, can be administered such as to produce levels of sedation that range from minimal sedation to moderate sedation/analgesia.

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GOALS OF SEDATION

The goal of sedation is to: (1) provide an optimal environment for completion of the surgical procedure, (2) minimize patient anxiety and optimize patient comfort, (3) control the patient’s behavior and movement and optimize patient cooperation, (4) optimize analgesia and minimize patient discomfort and pain, (5) maximize the potential for amnesia, and (6) optimize patient safety and maintain hemodynamic stability. Each situation (involving the patient and the specific surgical procedure) is independently evaluated to determine the depth or level of sedation required to achieve these goals.

The sedation of a child is different then that of an adult. For the young child, who lacks the coping capability and the social adaptability, the primary goal is to control the child’s behavior and movement and to optimize patient cooperation. This differs from that of an adult patient in which the primary goal is to minimize patient anxiety and optimize patient comfort. Because of the different goals, the child frequently will require a greater depth of anesthesia. Alternatively, if a moderate sedation or a deep sedation is used instead of a general anesthetic for the management of the pediatric patient, the practitioner will need to have a different level of tolerability. For example, the practitioner must be willing to accept some crying and possible movement that may require minimal restraint if a level of moderate sedation (or possibly deep sedation) is planned for a pediatric patient.

The practitioner must also select which medications that will be used to achieve the anesthetic plan. Although there may be specific medications, such as propofol or methohexital, which may more commonly be used to achieve an anesthetic depth of deep sedation or general anesthesia, all of the anesthetic drugs that are used for “lighter” levels of anesthesia, such as minimal sedation, can result in effects associated with deep sedation or general anesthesia. Alternatively, in appropriate doses propofol or methohexital can be titrated to achieve a level of moderate sedation. Ketamine has seen a resurgence in interest in the past decade. The dissociative state resulting from ketamine provides a favorable surgical environment in which the patient becomes relatively immobile while maintaining an intact and stable airway. The desire to incorporate the drug into every technique needs to be weighed against an understanding of the drug’s sympathetic effect. For example, nonjudicious use of the drug in a cardiovascularly compromised patient or a patient on a tricyclic antidepressant can have unfavorable consequences. In the former, the increased sympathetic stimulation can negatively impact the myocardial imbalance of oxygen supply and demand, and in the latter the patient may have prolonged sympathetic effects because of the interaction of the two drugs.

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GOALS OF SEDATION

The goal of sedation is to: (1) provide an optimal environment for completion of the surgical procedure, (2) minimize patient anxiety and optimize patient comfort, (3) control the patient’s behavior and movement and optimize patient cooperation, (4) optimize analgesia and minimize patient discomfort and pain, (5) maximize the potential for amnesia, and (6) optimize patient safety and maintain hemodynamic stability. Each situation (involving the patient and the specific surgical procedure) is independently evaluated to determine the depth or level of sedation required to achieve these goals.

The sedation of a child is different then that of an adult. For the young child, who lacks the coping capability and the social adaptability, the primary goal is to control the child’s behavior and movement and to optimize patient cooperation. This differs from that of an adult patient in which the primary goal is to minimize patient anxiety and optimize patient comfort. Because of the different goals, the child frequently will require a greater depth of anesthesia. Alternatively, if a moderate sedation or a deep sedation is used instead of a general anesthetic for the management of the pediatric patient, the practitioner will need to have a different level of tolerability. For example, the practitioner must be willing to accept some crying and possible movement that may require minimal restraint if a level of moderate sedation (or possibly deep sedation) is planned for a pediatric patient.

The practitioner must also select which medications that will be used to achieve the anesthetic plan. Although there may be specific medications, such as propofol or methohexital, which may more commonly be used to achieve an anesthetic depth of deep sedation or general anesthesia, all of the anesthetic drugs that are used for “lighter” levels of anesthesia, such as minimal sedation, can result in effects associated with deep sedation or general anesthesia. Alternatively, in appropriate doses propofol or methohexital can be titrated to achieve a level of moderate sedation. Ketamine has seen a resurgence in interest in the past decade. The dissociative state resulting from ketamine provides a favorable surgical environment in which the patient becomes relatively immobile while maintaining an intact and stable airway. The desire to incorporate the drug into every technique needs to be weighed against an understanding of the drug’s sympathetic effect. For example, nonjudicious use of the drug in a cardiovascularly compromised patient or a patient on a tricyclic antidepressant can have unfavorable consequences. In the former, the increased sympathetic stimulation can negatively impact the myocardial imbalance of oxygen supply and demand, and in the latter the patient may have prolonged sympathetic effects because of the interaction of the two drugs.

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CONCEPT OF RESCUE

There is a prevailing thought in medicine that human errors are inevitable and that these errors are not necessarily secondary to incompetence. An American Society of Anesthesiologists (ASA) closed claims analysis reported that up to 80% of anesthetic mishaps could have been prevented and were attributable to human error. The concept of rescue therefore is essential to safe sedation. The office-based environment cannot be dependent on emergency medical services (EMS) as the primary intervention for adverse events. The office must be equipped and the staff able to manage the most serious of adverse events. The first line in preventing an adverse event is prevention. This has two components: (1) preoperative assessment and (2) ensuring that the practitioner who is administering the anesthetic is trained and capable of rescuing the patient from a deeper level of sedation than was initially planned.

Preanesthetic assessment is the focus of a separate chapter. However, there are a few points that I would like to emphasize here. The preanesthetic or preoperative assessment is the process in which the practitioner ensures that the patient is fit for anesthesia and surgery. The aim of the assessment is to obtain a good medical history and complete a focused physical examination. The medical history alone reveals up to 90% of the needed information. Two distinct types of office-based surgery practices exist among our oral and maxillofacial surgery colleagues. Those who perform this preanesthetic assessment solely on the day of surgery and those who perform this on a separate appointment before the scheduled surgery date. There are a few advantages to the latter methodology: (1) the ability to assess the patient’s level of anxiety and provide psychological intervention either behaviorally or pharmacologically and (2) the ability to modify and direct treatment based on the patient’s medical history (e.g., management of long-term medication, such as what medications should be taken and what drugs not taken on the day of surgery).

Rescue therapies require continuous and repeated training to ensure a level of skill and knowledge. This is particularly important in an environment, such as the oral and maxillofacial surgery office, in which there is a rarity of a significant adverse event occurring. There are several measures that can be taken by the practitioner to prevent the occurrence of an adverse event or minimize the undesirable consequences of such an adverse event. The practice of ambulatory office-based anesthesia in the oral and maxillofacial surgeon’s office emphasizes team management. The concept of team management has been shown to optimize performance and minimize errors in medicine. Additionally the practitioner and the anesthetic team should prepare for anesthetic care similar to a flight crew. Using this comparison, the anesthetic team must be comfortable and familiar with the anesthetic equipment. If the equipment is new to their office, this may entail a more detailed review to ensure familiarization. If the equipment is older, although the office may be familiar and fairly comfortable with it, its inspection must focus on potential effects resulting from age and ensure that it remains optimally functional. This requires a protocol in which the equipment (both that used for routine care and emergency intervention) is regularly inspected and the anesthetic team has the opportunity to practice with it. The latter point emphasizes the other concept in training and performance testing among pilots. Pilots use simulators to artificially create a scenario that is analogous to real life. Simulators can provide scenarios that represent normal anesthetic management and emergency situations. The ability to represent emergency situations is of particular interest because many individuals may never encounter a particular emergency during training until they encounter such a situation during actual clinical patient care.

There are four different types of simulators: screen-based text simulators, screen-based graphical simulators, mannequinbased simulators, and virtual reality simulators. The mannequin-based simulator includes an anatomically correct physical model of a patient. The mannequins vary in complexity. The level of mannequin sophistication may allow the trainee to ventilate and/or intubate the mannequin, auscultate heart and lung sounds using anatomically placed speakers, assess respiratory efficacy by measuring end-tidal CO ₂ and pulse oximetry, palpate carotid and radial pulses, and establish intravenous (IV) access. These simulators have been shown to enhance skills without endangering the health of patients. They facilitate acquisition of knowledge and skills that are transferable back to clinical care. However, the mannequin simulators are expensive and lack easy portability. The screen-based graphical simulators attempt to visually recreate the clinical environment. The simulation programs provide physiologic data (such as HR, BP, ECG, O ₂ Sat, end-tidal CO ₂ ), provide patient interaction (such as patient responses to preset questions), allow responses to physical examination (such as breath or heart sounds or level of responsiveness), and provide realistic responses to drug administration based on the pharmacokinetic and pharmacodynamic properties of the medications. Two studies completed at the University of Washington demonstrated that the use of the screen-based graphical simulator demonstrated better retention and better application of skills and knowledge than those who used a text solely.

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AIRWAY ASSESSMENT

Assessment of the airway is one of the most critical components of the preoperative assessment. This is exemplified by two separate ASA closed claims analysis publications that reported respiratory adverse events as the most common mechanism of injury to the patient. Forty-one percent of these claims were for death or permanent brain damage. The incidence of respiratory adverse events, most of which were inadequate oxygenation/ventilation, was comparable between monitored anesthesia care (MAC) and general anesthesia. The first step in preventing these adverse respiratory events is to identify a potential difficult airway.

Airway assessment consists of both history and examination. The history obtained from the patient should inquire about prior surgeries on the airway or difficulties with airway management during prior surgeries. The examination component of the assessment focuses on several anatomic components to the airway that can predict difficult airway management: (1) modified Mallampati test, (2) thyromental distance, (3) mandibular retrognathia, (4) interincisal opening less than 3 cm, (5) short neck, (6) thick neck (increased neck circumference of 17 inches for males and 16 inches for females), and (7) diminished range of motion of head and neck with the inability to extend or flex the neck (such that the tip of the chin can touch the chest). The modified Mallampati test assesses the airway based on the ability to visualize the uvula and faucial pillars when the mouth is wide open, the tongue is maximally protruded, the neck is extended forward into the sniffing position, and the patient phonates. The modified Mallampati classification has five categories that are defined in Table 5-1 , with the higher class supposed to predict greater difficulty in intubation.

The thyromental distance is a measurement from the mandibular menton to the prominence of the thyroid cartilage with the neck in full extension. The distance should be at least 6 cm or approximately three ordinary fingerbreadths. These tests were initially proposed, and most studies to date have investigated their ability to predict difficult intubations. Used independently, none of these tests was sufficient to consistently predict the difficulty of airway intubation. Combining several different tests (such as the Mallampati test and the thyromental distance) to assess a patient’s intubatability did result in a higher sensitivity, higher positive predictive value, and fewer false-negative results. Because most office-based anesthesia for oral and maxillofacial surgery is done on the nonintubated patient, we are actually more interested in the ability to ventilate and to maintain airway patency than we are in the ability to intubate a patient. There are fewer studies assessing the ability to ventilate a patient or maintenance of airway patency. The incidence of being unable to ventilate a patient is approximately 5% of the general adult population. The Mallampati test was found to be no better in predicting difficult mask ventilation than difficult airway intubation. One publication did show a correlation in difficulty in mask ventilation among patients who were difficult to intubate.

Maintaining airway patency in the nonintubated patient is always potentially a risk because operating in the oral cavity can result in airway obstruction secondary to such factors as the positioning of the mandible, head-neck flexion-extension, or posterior displacement of the tongue. The above criteria provide some guidance as to the level of sedation, which would be appropriate or inappropriate for a particular patient in regard to the potential for airway difficulty. The surgeon may decide to manage the patient with significant findings either with a minimal sedation in the office or plan on using advanced airway techniques (either a laryngeal mask airway (LMA) or intubation) either in the office or hospital or surgery center. If the potential for a difficult airway is recognized, then the appropriate preparations can be made to increase the likelihood of a good outcome. The complexity with the ability to predict a difficult airway remains not with the extremes of the assessment criteria but with those patients who fall within the middle. Other factors, such as obesity, history of snoring, abnormal oropharyngeal or neck masses, prior airway surgeries, or congenital, developmental or acquired facial deformities (e.g., craniofacial syndromes, burns to the head and neck), may provide further guidance.

Obesity warrants further comment because its incidence is rapidly increasing in the United States with an incidence in children and adolescents that exceeds 15% and in adults that exceeds 30% and is predicted to approach 40% in 2008. Physical findings previously discussed that are found in the obese patient include: a decrease in cervical and mandibular range of motion, a decrease in thyromental distance, and a less favorable Mallampati class. The patient’s airway is more likely to collapse during anesthesia, and the distribution of tissue in the patient’s neck and chest will complicate airway management. This is compounded by the fact that the patient is more susceptible to developing hypoxemia or hypercapnia because of restrictive lung disease (an increased chest wall mass decreases chest wall compliance and diaphragmatic excursion), increased closing volume, and ventilation-perfusion mismatch. There is also a higher incidence of obstructive sleep apnea (OSA) in the obese patient. It is estimated that OSA is undiagnosed in 80% of patients who have the condition. This extends the concern with airway and respiratory management beyond the intraoperative period into the postoperative period. The literature is insufficient to evaluate the effects of various analgesic medications on OSA. These patients will benefit from multimodal analgesic techniques (e.g., preemptive NSAIDs) and minimization of opioids, and their respiratory depressant effects will most likely reduce the potential for adverse events. The ASA guidelines suggest that the “OSA patient be monitored for a median of three hours longer than their non-OSA counterparts before discharge from the facility.”

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LARYNGEAL MASK AIRWAY

Traditional anesthetic management for intraoral surgery is unique in that the surgical site is contiguous with the unprotected nonintubated airway. The patient is susceptible to airway obstruction, airway irritation, and hypoventilation or apnea. This can necessitate surgical interruption to manipulate the airway to eliminate the obstruction and establish airway patency. This may be as easy as readjusting the pharyngeal curtain or performing a head-tilt maneuver. Various interventions may also be required to remove airway irritation. These could likewise include repositioning the pharyngeal curtain, which could be an irritant to the pharynx, or suctioning the pharynx of some irrigating solution that may have passed beyond the pharyngeal curtain. Rarely the patient may require positive pressure ventilation.

The LMA forms an intermediate form of airway management between the face mask and the endotracheal tube. For intraoral surgery, it provides the ability to obtain, maintain, and secure a patent airway. The advantage of the LMA is that it is passed beyond the tongue, forming a seal with the laryngeal inlet and eliminating the most common cause of upper airway obstruction in the nonintubated patient. Because there is not a direct conduit into the trachea as achieved with endotracheal intubation, there is a potential for airway obstruction, which has been reported as high as 30%. This obstruction generally responds to head repositioning with the need to neither reposition the LMA nor interrupt surgery. Breathing may be spontaneous, assisted, or controlled with the LMA, avoiding the need to interrupt surgery to provide positive pressure ventilations. The LMA is also an excellent barrier against aspiration of irrigating solution, surgical debris, and blood. A pharyngeal curtain, however, is recommended to keep any surgical debris contained within the oral cavity and minimize the potential for aspiration of any surgical debris when the LMA is removed.

General anesthesia is indicated when using the LMA. Using the standard technique, the LMA is inserted blindly after induction of general anesthesia. The blind insertion technique, which requires less airway manipulation compared with intubation, is associated with less sympathetic stimulation and potential hemodynamic fluctuations. Campbell et al recommends laryngoscopy and direct visualization of LMA insertion as an alternative to optimize anatomic placement and minimize the epiglottis from impinging on the glottic opening. Theoretically, this should improve airflow dynamics. These authors reported that the brief laryngoscopy for LMA placement did not adversely result in sympathetic stimulation and hemodynamic changes. Neither the blind nor laryngoscopic insertion technique require neuromuscular blockade. The LMA is also advantageous compared with endotracheal intubation in that its anesthetic requirements both during maintenance and emergence are less.

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SMOKING

Approximately one-quarter of the American adult population smoke cigarettes. Cigarette smoking is a cause of increased perioperative morbidity. This may be secondary to the chronic diseases associated with smoking, such as ischemic heart disease and chronic obstructive pulmonary disease. The constituents of cigarette smoke, such as nicotine and carbon monoxide, also have acute effects on both the cardiovascular and respiratory systems.

From a cardiovascular perspective, nicotine activates the sympathoadrenergic system. This increases heart rate, blood pressure, and peripheral resistance and increases oxygen demand. Expired carbon monoxide levels have also been shown to correlate with ST depression under general anesthesia. Cigarette smoking also promotes a hypercoagulable state contributing to arterial thromboembolism and coronary vasospasm.

Effects on the respiratory system include impaired mucociliary clearance and mucous hypersecretion, which results in increased airway irritability. This may be manifested clinically by a higher incidence of laryngospasm and airway obstruction. Small airway narrowing contributes to increased closing capacity, which results in ventilation-perfusion mismatch. Cigarette smoking also increases the blood carbon monoxide level and a leftward shift in the oxygen-hemoglobin dissociation curve. The increase in the carboxyhemoglobin level (8% to 10%) results in less oxygen being carried by hemoglobin. The leftward shift in the oxygen-hemoglobin dissociation curve results in less oxygen being released to the peripheral tissues. The net effect is a decrease in tissue oxygenation. Passive smoke exposure has also been shown to be a risk factor in children contributing to airway complications (laryngospasm ∼5 times greater and airway obstruction ∼3 times greater) during the intraoperative and postoperative period.

Smoking can also have a detrimental effect on wound healing including wound dehiscence, infection, and impaired bone healing. This may be secondary to effects on the immune system or changes in tissue oxygenation.